possible mechanisms of acquisition of herpesvirus virokines · 2017. 3. 6. · structure of the...

Interaction between viruses and systems of host pro-

tection is crucial in the evolution of both viruses and

hosts. “Stealing” the gene encoding a component of the

host’s protection system is one of the major strategies

used by viruses for avoiding the host’s immunity control

[1]. During evolution, within the viral genome a protein

component of this gene can acquire new features, for

instance, it can become an inhibitor of the corresponding

protective system. Another strategy is also frequently real-

ized when a virus homolog retains the ability to transmit

the signal to non-infected cells involved in the antiviral

immune response. In this case, the virus uses the immune

system cells and achieves a success in retargeting the

immune response and activating signaling pathways,

which promote its survival and replication. The term

“virokines” is applied to homologs of animal hosts whose

genes are present in the viral genomes [2]. These proteins

usually provide a selective advantage for their new own-

ers, the viruses, during their life cycle. Mechanisms of

virus interaction with cell are under active study, and the

contribution of virokines to the development of diseases

are under active study, but their evolutionary history,

except high homology between the host and viral genes,

usually remains beyond the attention focus.

Herpesviruses (Herpesviridae family) were especially

successful in borrowing genes of mammalian cytokines.

The genome of herpesviruses is double-stranded DNA

with length over 110 kb and contains about a hundred

genes, the majority of which encode proteins that are nec-

essary for avoiding the immune response, and frequently

are “stolen” from the host [3]. This arsenal of immune

regulators allows herpesviruses to exist for a long time in

the host organism as a latent chronic infection. However,

malignant transformation of the cells can be a side effect

of the virus introduction into protective mechanisms.

The mechanism of borrowing genes use by her-

pesviruses needs separate consideration. The most obvious

“know-how” of copying the host’s genes is recombination

between the viral and host genomes during virus replica-

tion within the nucleus. A latent virus can also use nonho-

mologous recombination for repair of spontaneous dou-

ISSN 0006-2979, Biochemistry (Moscow), 2016, Vol. 81, No. 11, pp. 1350-1357. © Pleiades Publishing, Ltd., 2016.

Original Russian Text © E. A. Gorshkova, E. S. Shilov, 2016, published in Biokhimiya, 2016, Vol. 81, No. 11, pp. 1604-1613.

1350

Abbreviations: BaLCV, baboon lymphocryptovirus; Bonobo-

HV, bonobo herpesvirus; EBV, Epstein–Barr virus; HHV,

human herpesvirus; HVS, herpesvirus saimiri; IFN, interferon;

IL, interleukin; MRV, rhesus macaque rhadinovirus; RhCMV,

rhesus macaque cytomegalovirus; RhLCV, rhesus macaque

lymphocryptovirus; SaHV-2, Saimiriine herpesvirus 2; vIL,

virokine, interleukin homolog.

* To whom correspondence should be addressed.

Possible Mechanisms of Acquisition of Herpesvirus Virokines

E. A. Gorshkova1,2 and E. S. Shilov1,2*

1Lomonosov Moscow State University, Faculty of Biology, 119991 Moscow, Russia2Engelhardt Institute of Molecular Biology, Russian Academy of Sciences,

119991 Moscow, Russia; E-mail: [email protected]

Received June 17, 2016

Revision received July 29, 2016

Abstract—The genomes of certain types of human and primate herpesviruses contain functional homologs of important host

cytokines (IL-6, IL-17, and IL-10), or so-called virokines. Virokines can interact with immune cell receptors, transmit a

signal to them, and thus switch the type of immune response that facilitates viral infection development. In this work, we

have summarized possible ways of virokine origin and proposed an evolutionary scenario of virokine acquisition with

involvement of retroviral coinfection of the host. This scenario is probably valid for vIL-6 of HHV-8 and MRV-5 viruses,

vIL-17 of HVS virus, and vIL-10 of HHV-4, Bonobo-HV, RhLCV, and BaLCV viruses. The ability to acquire cytokine

genes allows herpesviruses to implement unique strategies of avoiding the immune response and provides them an evolu-

tionary advantage: more than 90% of the host population can be chronically infected with different herpesviruses. It is pos-

sible that the biological success of herpesviruses can be partially due to their cooperation with another group of viruses. This

hypothesis emphasizes the importance of studies on the reciprocal influence of pathogens on their coinfection, as well as

their impact on the host organism.

DOI: 10.1134/S0006297916110122

Key words: virokines, IL-6, IL-10, IL-17, herpesviruses, retroviruses, horizontal gene transfer

ORIGIN OF HERPESVIRUS VIROKINES 1351

BIOCHEMISTRY (Moscow) Vol. 81 No. 11 2016

ble-stranded breaks: genomes of herpesviruses are found

to contain many specific sequences detectable by the cel-

lular repair systems [4]. In this case, the exon–intron

structure of the “stolen” gene must be preserved and fol-

lowed in the virus homologs. However, introns are present

not in all genes borrowed from the host, thus only a quar-

ter of genes of human herpesvirus 8 (HHV-8) contains

introns [5]. It was reasonable to suppose that introns could

be lost because of selection pressure, because there were

also observed “transitional variants” of the structure, i.e.

shortened introns or intron sets incomplete relatively to

the cellular gene. However, another scenario also seems

probable: the gene can enter the virus already in a spliced

state without introns. This probability is indirectly con-

firmed by consideration of the gene regions on the border

between the exon and intron. If the intron had been lost

because of an occasional deletion, many insertions and

deletions within the reading frame would have occurred in

the beginning and ends of the virus gene regions homolo-

gous to the initial gene exons. These insertions and dele-

tions would be caused by inaccurate removal of the intron

at the splicing borders [6]. In reality, in the majority of

cases of intron removal, the reading frame is neither elon-

gated nor shortened; consequently, the DNA sequence of

the gene really enters the viral genome in the spliced state

due to reverse transcription. Herpesviruses themselves

cannot realize reverse transcription, and endogenous or

exogenous retrovirus acts as a mediator infecting the same

cells as the herpesvirus.

One has to understand that the initial gene of the cel-

lular cytokine can enter the herpesvirus genome also

directly, without involvement of retroelements, but under

the influence of systems responsible for repair and nonho-

mologous recombination. In this case, the virokine gene

will retain the initial exon–intron structure. For one-exon

genes of virokines, we can suppose that a herpesvirus can

borrow genes using several pathways (Fig. 1): (i) the activ-

ity of endogenous retroviruses in the germ line cells leads

to origination in the host’s genome of processed pseudo-

genes, and after that the herpesvirus acquires a copy-

pseudogene in the infected cells as a result of nonhomolo-

gous recombination with the host’s genome; (ii) an

endogenous retrotransposon incorporates cDNA of the

gene into the genome of a latent herpesvirus directly in the

infected target cell; (iii) the reverse transcription and

incorporation into the herpes viral genome are realized by

a retrovirus co-infecting the target cell pre-infected with

herpesvirus. In the present work, we compared these

hypothetical scenarios of virokine acquisition by the

known herpesvirus homologs IL-6, IL-17, and IL-10. For

each virokine, we determined the systematic position of a

host of the virus as the hypothetical donor of the virokine

gene, performed a search for pseudogenes of the

cytokines, and analyzed the probability of expression of

endogenous and exogenous retroviruses in cells infected

by herpesviruses to show the most probable scenario.

MATERIALS AND METHODS

Nucleotide and amino acid sequences of the

virokines were taken from annotated genomes of her-

pesviruses (human herpesvirus 8, Saimiriine herpesvirus 2,

and human herpesvirus 4), nucleotide and amino acid

sequences of cytokines IL-6, IL-17, and IL-10 of primates

(Homo sapiens, Pan troglodytes, Gorilla gorilla, Pongo

abelii, Macaca mulatta, Papio anubis, Chlorocebus sabaeus,

Callithrix jacchus, Saimiri boliviensis, Aotus nancymaae,

Microcebus murinus, and Otolemur garnettii) and of rodents

(Mus musculus and Rattus norvegicus) were taken from

annotated genomes of the corresponding organisms. The

list of identification numbers on the NCBI site for protein

sequences of all organisms is presented in the table in the

Supplement to this report on the site of the journal

(http://protein.bio.msu.ru/biokhimiya) and Springer site

(Link.springer.com). The percent of identical amino acids

for the global alignment of sequences of virokines and of

their cellular homologs were calculated using the

EMBOSS Needle algorithm [7].

Multiple alignment of amino acid sequences of

virokines and their homologs was performed using the

MAFFT algorithm [8, 9]. Based on the multiple align-

ment, for each cytokine the most relevant evolutionary

model was chosen, and then by the maximum likelihood

method a phylogenetic tree was reconstructed based on

this model. Stability of the tree topology was tested by

bootstrapping (100 repeats). Portions of the amino acid

sequences with incompletely covered alignments

(“gaps”) were excluded from the analysis. Rodents were

used as an outgroup for primates. The best evolutionary

model was chosen and the phylogeny was reconstructed

by the maximum likelihood method in the MEGA6 pro-

gram [10]. Phylogenetic trees for every virokine were also

constructed by Bayesian analysis with a relaxed molecular

clock using the BEAST software package [11]. The trees

were constructed using the same evolutionary models as

for the reconstruction by the maximum likelihood

method, and at least two launchings were performed of

the algorithm with chain length of 10 million.

The search for sequences of pseudogenes of IL-6,

IL-17, and IL-10 cytokines was performed in the human,

rhesus macaque, and Saimiri genomes using BLAT

instruments [12] and blastn at the NCBI site. The time of

homolog divergence was assessed based on the divergence

time of the taxons using the TimeTree service

(http://timetree.org/) [13].

RESULTS

To determine the probable mechanism of virokine

origin, we analyzed several genes of herpes virokines for

the presence of introns and for the presence in the pres-

ent-day host species genome of intron-free pseudogenes,

1352 GORSHKOVA, SHILOV


Fig. 1. Three probable scenarios of virokine gene acquisition by herpesvirus.



and for the type of target cells and the presence of retro-

viruses co-infecting the same targets. These data and the

supposed existence time and the systematic position of

the hosts as of virokines sources are presented in the table

(for brevity, only three from twelve IL-10 homologs are

presented). We analyzed the present-day herpesvirus

virokines from the standpoint of mechanisms of their

acquisition and also of donor genomes and established

that the most probable source of the viral IL-6 was a

common ancestor of the dry-nosed monkeys

(Haplorhini), of viral IL-17 – a common ancestor of the

flat-nosed monkeys (Platyrrhini), and of viral IL-10 of

HHV-4 – an ancestor of apes (Hominidae). Specific fea-

tures of evolution and clinical significance of individual

virokines are considered later in the corresponding sub-

sections.

Viral IL-6. Homologs of IL-6 are present in the

genome of human herpesvirus 8 (HHV-8), which is also

known as Kaposi’s sarcoma-associated herpesvirus, and

in rhadinoviruses of some macaque species, especially of

rhesus macaque (MRV-5). MRV-5 belongs to the so-

called RV2-line of monkey rhadinoviruses that also

includes rhadinovirus of the pig-tailed macaque

(MneRV2), Japanese macaque (MfuRV2), and long-

tailed macaque (MfaRV2). Together with herpesviruses of

macaque retroperitoneal fibromatosis (RFHV) and with

rhadinoviruses of other species of the Old World monkeys

(chimpanzee, gorilla), HHV-8 forms the RV1-lineage,

which is different from RV2 by the presence of additional

genes. Viral IL-6 can be found in all human and macaque

viruses belonging to the RV1- and RV2-lineages, but it is

absent in rhadinoviruses of other monkey species [17].

Homologs of IL-6 of rhadinoviruses are free of introns.

B-cells are the major reservoir of a latent HHV-8 infec-

tion with expressed early viral genes responsible for

avoiding apoptosis and cytotoxic immunity and for inhi-

bition of production of antiviral cytokines. The viral IL-6

acts in an autocrine manner, increasing cell viability and

inducing expression of its IL-6 [18]. The virokine can

suppress the cellular antiviral defense due to inhibition of

IFNα production, but it should be noted that the viral IL-

6 gene promoter contains an IFNα-dependent element,

thus vIL-6 regulates the host’s IFNα through negative

feedback [19]. The etiological role of vIL-6 has been

established for such tumors as primary effusion lym-

phoma (PEL) and multicentric Castleman disease

(MCD). The viral IL-6 can also cause development of

syndrome of inflammatory cytokines that are associated

with HHV-8 due to increasing production of the human

IL-6, which leads to cytokine storm and systemic inflam-

mation [20]. Rhadinovirus of rhesus macaque is a natural

infectious agent – about 90% of animals of these species

are shown to be seropositive for this virus in two American

regional centers of studies on primates. However, onco-

logical diseases similar to human PEL and MCD seldom

appear in them; usually they develop on decrease in

immune defense caused by simian immunodeficiency

virus (SIV) [21]. Nevertheless, rhadinovirus infection

during coinfection with SIV is the most relevant model of

Kaposi’s sarcoma in AIDS-positive patients [22].

Herpesvirus

Virokine

Number of exons/introns

Host

Cytokine pseudo-genes in the host’s genome

Target cells

Co-infecting retroviruses

Hypothetical hostas virokine source

Characteristics of herpesvirus virokines

RhCMV

cmvIL-10

4/3

rhesus macaque

absent

В-cells

SRV-1 [14]

a common ances-tor of the Old andNew World mon-keys, >42 millionyears ago [16](Fig. 2)

HHV-4 (EBV)

vIL-10

1/0

human

absent

В-cells

unknown

a commonancestor of allhominids, >15million years ago(Fig. 2)

MRV-5

vIL-6

1/0

rhesus macaque

absent

В-cells

SRV-1 [14]

a commonancestor of theOld and NewWorld mon-keys, >42 mil-lion years ago(Fig. 2)

HHV-8

vIL-6

1/0

human

absent

В-cells

unknown

a commonancestor ofthe Old andNew Worldmonkeys, >42million yearsago (Fig. 2)

HVS (SaHV2)

vIL-17

1/0

squirrel monkey(Saimiri)

absent

Т-cells

SIV [15]

a commonancestor of theNew Worldmonkeys, <42million yearsago (Fig. 2)

HHV-5 (CMV)

cmvIL-10

3/2

human

absent

В-cells

unknown

a commonancestor of theOld and NewWorld monkeys,>42 million yearsago [16] (Fig. 2)



The amino acid sequence of vIL-6 (open reading

frame K2 HHV-8) has only 25% identity with the

sequence of human IL-6, but it has likeness on the level of

the secondary and tertiary structure: the virokine has a

motif containing four α-helices that is specific for all

cytokines of this family. However, the vIL-6 virokine bind-

ing with cellular receptors has some specific features lack-

ing in other proteins of the family [23]. As discriminated

from the cellular IL-6, which binds with a specific high

affinity IL-6R receptor and already within the complex

interacts with gp130 molecule [24] transmitting into the

cell the signal from the receptor through the molecular

cascade JAK/STAT, the virokine can bind directly with

gp130, producing tetrameric signaling complexes (2vIL-

6–2gp130) [25]. However, for transmitting the signal, the

virokine concentration has to be significantly higher than

values characteristic for the cellular IL-6 [26].

The amino acid sequence of an IL-6 homolog from

the rhesus macaque rhadinovirus has 36% identity with

the host’s cytokine and 27% identity with K2 HHV-8;

therefore, it can be supposed that the mechanism of inter-

action of this virokine with cellular receptors should be

similar to that of the HHV-8 virokine. Phylogenetic

analysis of the amino acid sequences of these two

virokines, as well as of IL-6 from cytokines of primates,

revealed (Fig. S1 of Supplement) that both cytokines are

clustered into a separate branch located between the

branch of the wet-nosed monkeys (lemurs and lorises) and

the branch of dry-nosed monkeys (Haplorhini) (including

flat-nosed (Platyrrhini) and narrow-nosed (Catarrhini)

monkeys). According to literature data, the divergence of

HHV-8 and MRV-5 viruses occurred ~38 million years

ago [27], earlier than the divergence of human and rhesus

macaque evolutionary branches (28 million years ago), but

later than the divergence of flat-nosed and narrow-nosed

monkeys (42 million years ago). Thus, virokines homolo-

gous to IL-6 belong to the most ancient herpesvirus

virokines. We suppose that a common ancestor of HHV-8

and MRV-5 viruses received the gene of this cytokine from

a common ancestor of dry-nosed monkeys who lived ear-

lier than 42 million years ago, before the divergence of the

branches of the Old and New World monkeys. The high

divergence degree between two virokines suggests that the

divergence of hosts was accompanied by divergence of

viruses, which did not change the specificity, and evolu-

tion of the viruses was parallel to evolution of the hosts.

Viral IL-17. The IL-17 homolog has been found only

in the genome of Saimiriine herpesvirus 2 (HVS or SaHV-

2), which also belongs to rhadinoviruses and is free of

introns. For squirrel saimiri, HVS is a natural infectious

agent, whereas for other primates (tamarins, marmosets,

macaques) this virus can cause malignant T-cell diseases.

It has been shown that the HVS-C virus lineage is capable

of transforming human T-cells [28]. The etiological role

of the viral IL-17 has been established for human idio-

pathic pulmonary fibrosis [29].

The amino acid sequence of viral IL-17 (ORF13 of

Saimiriine herpesvirus 2) has 74% identity with IL-17А of

saimiri and 71% identity with human IL-17А, and the viral

homolog can bind to the cellular receptor IL-17RA. Note

that the IL-17 receptor of mouse IL-17RA was first identi-

fied and cloned just due to its binding with vIL-17 [30]. It

was established later that in animals the receptor is usually

present as an IL-17RA/IL17RC heterodimer and can

interact with the homodimeric cellular cytokine IL-17A.

The structure of the receptor complex has not been studied

in detail, but the likeness of pictures of the tissue damage in

chronic pulmonary diseases contributed by IL-17 and in

idiopathic pulmonary fibrosis associated with the SaHV-2

infection indirectly confirms that the viral IL-17 not only

binds a subunit of the receptor, but also transmits a proin-

flammatory signal similar to the cellular one [29, 31].

Analysis of six IL-17 homologs known in mammals

revealed that the cellular IL-17A is the closest to the

virokine, which is why we have taken for the phylogenet-

ic analysis the sequences only of IL-17А genes of primates

and rodents. On the phylogenetic tree, this virokine is

clustered in the same branch with cytokines of flat-nosed

monkeys (Fig. S2 of Supplement), but is significantly

remote from them. Therefore, its acquisition can be con-

sidered a rather early event; it most probably occurred

soon after the divergence of flat-nosed and narrow-nosed

monkeys (42 million years ago).

Viral IL-10. Homologs of IL-10 have been found in

more than twenty viruses belonging to three different fami-

lies: 12 viruses of Herpesviridae, seven viruses of Poxviridae,

and two viruses of Alloherpesviridae. Such wide distribution

of IL-10 homologs among different families of viruses indi-

cates a significant advantage acquired by a virus capable of

producing immunosuppressive cytokine in the infected cells

and evidences independent borrowing of the IL-10 gene

during evolution, which could have occurred through dif-

ferent mechanisms. Thus, the best studied IL-10 homolog

of human herpesvirus 4 (Epstein–Barr virus, HHV-4) and

of the related species that also occur in other primates

(bonobo herpesvirus (Bonobo-HV), rhesus macaque lym-

phocryptovirus (RhLCV), baboon lymphocryptovirus

(BaLCV)) has over 90% identity with the host’s gene

sequence and does not contain introns. However, homologs

of IL-10 found in human and monkey cytomegaloviruses

contain from one to three introns and encode proteins with

amino acid sequences that have only 20-30% identity with

the initial cytokine sequence. In addition, virokines with

amino acid sequences possessing both high and low similar-

ity degree with the cellular cytokines are capable of inter-

acting with cellular receptors of IL-10R1 due to the conser-

vative tertiary structure [32]. The phylogenetic tree for

sequences of the typical intron-free virokine IL-10 of apes

encoded by the BCRF1 locus of the HHV-4 virus indicates

(Fig. S3 of Supplement) that the virokine gene donor was a

member of Hominidae family and lived before their diver-

gence began, i.e. ∼15 million years ago.



DISCUSSION

Analysis of probable mechanisms of virokine acquisi-

tion. Evolution of herpesviruses closely followed evolu-

tion of their hosts and was accompanied by repeated

transfers of virokine genes. Information about the most

probable time of these transfers obtained in our work and

taken from the literature is presented in Fig. 2.

Among the hosts’ genes, not only genes of cytokines

were acquired by herpesviruses. Thus, the HHV-8 virus

has genes of thymidine kinase, thymidylate synthase, and

dihydrofolate reductase, which are necessary for synthesis

of pyrimidine nucleotides, ubiquitin ligases K3 and K5,

etc. It is interesting that the genes of herpesviruses

acquired by horizontal transfer are often linked between

themselves: the above-mentioned genes of the herpesvirus

IL-6, thymidylate synthase, and dihydrofolate reductase

in the genome of HHV-8 are located within a small area

with 4 kb size. The herpesvirus saimiri in the genome

homologous region instead of the proinflammatory

virokine IL-6 gene has the gene of another proinflamma-

tory virokine IL-17, which indirectly suggests the exis-

tence of similar patterns of regulation that are required by

viruses for successful use of proinflammatory cytokines.

Fig. 2. Evolution of herpesviruses was collinear to evolution of their hosts and was accompanied by repeated transfer of virokine genes from

the host to the virus. The arrows show the virokine gene transfer from host to herpesvirus.

Fig. 3. Schematic imaging of genomes of herpesviruses carrying virokine genes. Light-gray rectangles with Latin letters A-F indicate diver-

gent loci of herpesviruses, between them there are conservative genes specific for all herpesviruses. Above them, protein open reading frames

are shown as arrows: black ones – from left to right, white ones – from right to left. In rhadinoviruses (HHV-8 and SaHV-2), virokines are

located in the divergent locus B. The viral IL-10 is located in the divergent locus E of HHV-4 virus (Epstein–Barr virus, EBV). After data of

Nicolas et al. [33] (with modifications).

Papio anubis



Genomes of certain herpesviruses possessing virokines

are schematically presented in Fig. 3.

We searched for pseudogenes of cytokines IL-6, IL-

17, and IL-10 in the sequenced genome of present-day

primates using BLAT and blastn algorithms, but did not

find any homologous sequence with length above several

tens of nucleotides. It seems that they were absent also in

the genomes of extinct primates who were donors of the

virokine genes. This favors the hypothesis that the genes

IL-6, IL-17, and IL-10 “were stolen” by herpesviruses in

target cells with involvement of retroviruses or endogenous

retroviruses (the second and third mechanisms shown in

Fig. 1). Retroviruses of primates that infect lymphoid tis-

sue, first, T-cells (viruses of immunodeficiency and T-cell

leukemia) and less frequently, В-cells (retrovirus 1 of pri-

mates), are widely distributed and well known. Expression

of endogenous retroviral reverse transcriptase in lymphoid

tissue is poorly studied, but because the majority of retro-

transposons are located in the heterochromatin part of the

genome, it can be expected that their activity in differen-

tiated cells is a rare event associated with the entrance of

retroelements into introns of the genes with a high level of

tissue-specific expression. Thus, expression of some kinds

of HERV-K retrotransposons is described in human

peripheral blood lymphoid cells, but for the expression

they need to be specifically activated [34]. Nevertheless,

coinfection of the same host by retrovirus and herpesvirus

is a regular event reproduced many times during evolution.

Thus, it has been shown recently that genes of specific

retroviral superantigens nonspecifically activating T-cell

receptors can be transferred horizontally into the genome

of herpesviruses beyond the context associated with the

major histocompatibility complex (MHC) molecule of the

peptide [35]. Such molecules nonspecifically activate T-

cell immunity and thus prevent its targeting just the viral

antigens, i.e. partially act like proinflammatory cytokines.

The possibility of horizontal transfer of genes regulating

immune reactions of the host between given groups of

viruses is favorable for the hypothesis about arising of

virokine genes in the course of coinfection with retrovirus-

es. It is interesting that IL-6 promotes the polarization of

T-lymphocytes to Th17 and suppresses the polarization to

regulatory T-cells [36]. Thus, both types of proinflamma-

tory herpesvirus virokines, vIL-6 and vIL-17, act through

the same mechanism.

Having in mind the above-mentioned information,

the horizontal transfer from host to virus in the target cell

due to retroviral revertase activity seems probable for

intron-free genes of lymphotropic herpesvirus virokines:

vIL-6 of HHV-8 and MRV-5 viruses, vIL-17 of SaHV-2

virus, and vIL-10 of HHV-4, Bonobo-HV, RhLCV, and

BaLCV viruses. For intronized homologs of herpesvirus

vIL-10, the transfer into the viral genome is probable

through the nonhomologous recombination system.

However, the probability of such transfer of genes from

host to a virus is high only in the scale of evolution,

whereas it is extremely low for an individual case of her-

pesvirus infection. From this standpoint, it is interesting

to attempt to assess experimentally in vitro the frequency

of transfer of new host’s genes into the genome of a her-

pesvirus. In the easiest case, one can use a cell line con-

currently coexpressing GFP (a marker gene) and revertase

(analog of coinfection with retrovirus).

Advantages gained by a virus after acquisition of the

cytokine gene are associated either with suppression of

the immune response due to immunosuppressive IL-10,

or with switching this response to Th17- axis, which is

safe for the virus under the influence of proinflammatory

cytokines. Since virokines significantly contribute to

pathogenesis of some diseases associated with her-

pesviruses, it is clinically important to inhibit them, and

this approach is already used. Thus, different inhibitors of

the IL-6 signaling pathway are successfully used for treat-

ment of the lymphoproliferative Castleman’s disease [37].

Although cellular receptors are capable to of discriminat-

ing virokines and true cytokines by their affinity that

determines the difference in the effective working con-

centration [26], arising of an ideal system of cytokine sig-

naling protected against cytokines seems to be impossi-

ble. Considering the higher rate of evolution of her-

pesvirus genes and the significance of virokines for their

successful infection, from the “Red Queen hypothesis”

standpoint, cytokines and their receptors will not be able

“to break away” from virokines in the evolutionary race.

However, significant differences in the primary sequences

of virokines and their cellular prototypes allow us to

expect that discriminating epitopes should exist, allowing

the production of selective virokine-directed antibodies

and cytolysis of virokine-expressing cells.

Acknowledgements

We are grateful to S. A. Nedospasov for fruitful dis-

cussion of the material and valuable remarks.

The work was supported by the Russian Science

Foundation (project No. 14-25-00160).

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